Lasers utilizing an alkali vapor as the lasing medium have great potential as very high power lasers for military, industrial and scientific applications [Krupke et al, “New class of cw high-power diode pumped alkali lasers (DPALs)”, Proc. SPIE 5448, p. 156 (2004)].
Their high efficiency (related to a small quantum defect) and ability to remove heat and reduce pump induced distortions by flowing the vapor through the pumped/lasing region hold promise for highly efficient very powerful lasers. They can be pumped by efficient laser diode arrays.
The alkali vapor is contained in a cell, a portion of which is within the laser cavity. The laser cavity is defined by the lasing medium, an output coupler (OC), also referred to as a partial reflector, and a mirror high reflector (HR). The HR and/or OC may be incorporated in the walls of the cell with the alkali vapors or the cell may contain one or two windows through which the laser beam travels.
Typically the pump energy for these lasers is supplied by multiple laser diodes. This pump light may by itself be intense enough to damage optical components in the laser.
In the course of scaling these lasers to higher powers it became evident that the highly reactive alkali vapors were chemically attacking the outer layer of the thin film dielectric coatings used for antireflection (AR) coated windows, partially reflecting output couplers (OC) or highly reflecting mirrors (HR) when these components were incorporated into the walls of the cell. The chemical attack leaves deposits on the surface. The deposits are heated by the laser and/or pump beam(s), causing the surface to damage and extinguish the laser output [Zweiback et al, “28 W average power hydrocarbon-free rubidium diode pumped alkali laser” Optics Express 18, pp 1444-1449 (2010)]. Prior art efforts have been directed at choosing or developing bulk or coating materials that resist alkali attack or are only slowly attacked. However, none of the materials have proven to be fully satisfactory for long term operation when in direct contact with alkali vapors.
High power lasers (HPLs) often achieve powers so high that their optical components are heated by their residual (<0.1% or 1000 ppm) absorption of the laser beam. Since the heating is not uniform across a diameter, the laser beam's wave front is distorted from the non-uniform change in the shape of the component. For example, a one cm thick window with an absorption coefficient of only 0.0005 cm−1, transmitting a 100 kW/cm2 beam, is heated with 50 W/cm2 due to residual absorption. This grossly distorts the beam transmitted through the window and may fracture some windows. In extreme cases the component may be catastrophically fractured or melted, terminating the laser output. It is crucial that the absorption of optical components for HPLs have minimal (typically, less than 10 ppm) absorption.
Sapphire (Al2O3) and silica (SiO2) are used as common alkali vapor laser window materials and optical component substrates. Both are highly transparent and commercially available. Sapphire has greater strength and scratch resistance, while silica has a lower thermal expansion coefficient to reduce thermal stress.
In a diode pumped alkali vapor laser (DPAL) there is additional heating of the vapor cell windows or other optical elements in contact with the alkali vapor because the vapor is heated, typically to greater than 200° C., which heats any window in contact with it. In addition, chemical reactions between the alkali and the window/coating may add heat and/or leave a deposit that is heated by the laser and/or pump beam(s). Window contamination/laser damage issues have arisen during the development and scaling up of these lasers to high powers [see Zweibeck et al, Optics Express 18, pp. 14445-1449 (2010)].
The literature refers to a variety of alkali vapor cell window contamination issues, including impurities common to gas cells and gas circulation systems, decomposition products (largely eliminated by using He as the buffer gas instead of a hydrocarbon), adsorbed alkali metals, alkali metal oxides and alkali metal hydroxides/hydrates. Not only must permanent physical damage to the laser components be avoided, it is also necessary to avoid contamination that degrades the performance of the laser by refracting (thermal lensing), scattering, reflecting or absorbing light that should instead be contributing to the output of the laser. Windows examined after lasing may be contaminated by substances that were not present during lasing. For example, alkali metal atoms may react with a metal oxide surface to form an alkali oxide, but upon opening the chamber to the atmosphere, that alkali oxide may be converted to an alkali hydroxide/hydrate.
Laser damage of the optical components exposed to both an alkali vapor and the alkali laser pump beam or laser beam is limiting the power scaling and advancement of alkali vapor lasers. Clearly, a coating material that was less prone to chemical attack and/or catastrophic optical damage would represent a substantial improvement in the developing field of alkali vapor lasers.
See also U.S. Pat. No. 4,685,110 and U.S. Pub No.: 2012/02002031.